diff --git a/src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php b/src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php
index 8cc114ab2922..30c6f6ffab95 100644
--- a/src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php
+++ b/src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php
@@ -129,13 +129,13 @@ private function addTwigOptions(ArrayNodeDefinition $rootNode)
                 ->end()
                 ->scalarNode('autoescape_service')->defaultNull()->end()
                 ->scalarNode('autoescape_service_method')->defaultNull()->end()
-                ->scalarNode('base_template_class')->example('Twig_Template')->end()
+                ->scalarNode('base_template_class')->example('Twig_Template')->cannotBeEmpty()->end()
                 ->scalarNode('cache')->defaultValue('%kernel.cache_dir%/twig')->end()
                 ->scalarNode('charset')->defaultValue('%kernel.charset%')->end()
-                ->scalarNode('debug')->defaultValue('%kernel.debug%')->end()
-                ->scalarNode('strict_variables')->end()
+                ->booleanNode('debug')->defaultValue('%kernel.debug%')->end()
+                ->booleanNode('strict_variables')->end()
                 ->scalarNode('auto_reload')->end()
-                ->scalarNode('optimizations')->end()
+                ->integerNode('optimizations')->min(-1)->end()
                 ->arrayNode('paths')
                     ->normalizeKeys(false)
                     ->useAttributeAsKey('paths')